Cognitive Neuroscience Lecture 5: Visual Systems/Pathways
L5: Visual systems/pathways Visual system - Optics/eyeballs - Retina - Pathways - Cortex Overview of the visual pathway - There is energy in the world, in the form of light o Light is an electromagnetic wave o Light travels in a straight line o We see objects because they reflect light § But light reflects in all directions from each point of objects § And it arrives from all points § So the pattern of light and dark is destroyed o Solution: pinhole camera: select one direction per point § The problem with this is that when the pinhole is too small, not enough light § When the pinhole is too large, it is too blurry o Speed of light is slower in some materials than others o Refraction: § Because it travels at a diff speed in diff materials, light bends when crossing the boundary between materials § The refraction is proportional to the ratio of speeds o Prisms: divergent > <, convergent <> o Lenses § Light from objects scatters in many directions § Lenses gather light from a single point in world and focuses it on a single point on the screen (at a certain distance beyond the lens, say on a screen, or your retina) o Human eye § Ciliary body: muscles that control the shape of the lens § Cornea: most of the focusing power § Lens: mainly for fine tuning § Retina: no blood vessels on cornea or lens, but plenty of blood vessels right on retina · We don’t see them because visual system interested in change · If you were to shine a bright light, you’d see the shadow of the retina § Fovea: blood vessels and nerves pushed away, densely packed receptors, cones only o Accommodation (focusing) § Emmetropia: normal § Myopia: nearsightedness § Hyperopia: farsightedness - Light enters the eye, an image is formed on the retina o The retina has receptors that react to light and convert into neural signal to use in the brain o Photoreceptors: rods and cones o Cones = fovea § Daylight, packed in fovea, good for high-res vision and color § Short wavelength, medium wavelength, and long wavelength § S:M:L = 1:5:10 § Dearth of blue receptors in fovea (middle of image) § Retinal color blindness: lose L cones (protanopia) or M cones (deuteranopia) o Rods != fovea § Night time, not in fovea, everywhere but blind spot, low-res vision, no color, good for low-light conditions o NO RECEPTORS AT ALL in blind spot - Retina has receptors that react to light and transmit signals to the brain o Convergence of cones and rods § In fovea: low convergence (one to one mapping; high spatial res, high acuity) § In periphery: high convergence (many to one mapping; low spatial res, low acuity) · 1 RGC à many bipolar à many more rods o Optic nerve § No photoreceptors: blind spot § All axons from RGC come together § Form optic nerve bundle and exit eye - Signals get transmitted down the visual pathway from eye to cortex o Retina à optic tract à thalamus LGN à V1/striate cortex o More depth: retina àoptic nerve àoptic chiasmà optic tractàLGNàV1 o Your visual system is “cross-wired” § Signals from nasal half of each retina cross at the optic chiasm § Left VF àright v1 § Right VFàleft v1 § NOT ‘left eye to right cortex, right eye to left cortex’ § “what happens if”….do it the way you did, but then switch left to left after o Subcortical waystations § LGN receives 90% of retinal input and transmits info to cortex § Superior colliculus receives remaining retinal input, important for eye movements o Parallel pathways § Two types of RGC: magnocellular (rods), parvocellular (cones) § Temporal frequency higher for magnocellular - The brain tries to determine what is where in the world o Receptive fields: the area over which stimulation affects the response of a cell § \mapping a receptive field: allowed recording from otherwise normal animals § Use micro-electrode § Screen, amplifier, loudspeaker, oscilloscope, micro-electrode, anesthetize animal o What makes a V1 neuron fire? Hubel & Wiesel § Take a cell in v1 § Stimulate photoreceptors on retina § Determine which pattern of light the neuron fires for § Talk about a needle in a haystack! § They found out it was straight lines/bars of light § Simple v1 cells: bar of light o Receptive fields in RGC: center-surround (‘Mexican Hat’) § Not just single spot, small region with concentric ON/OFF responses § On/off = whether middle region “likes” light or dark § Excitation and inhibition cancel out if diffuse light, and NEITHER turns on if 0 light o What does the retina care about? § Sensitive to changes in local contrast § Only when it doesn’t cancel (light only hits concentric circle or middle ) § Retina signals edges, not uniform luminance § Sensitive to changes in local contrast : exaggerates it (actual luminance distribution is more sudden than perceived luminance distribution) o Not only stronger responses § ON: neuron fires more when light shown § OFF: neuron stops firing when light shown o V1: calcarine fissure § Adjacent regions of space are coded for by adjacent neurons § Retinotopic mapping in humans with fMRI (polar and eccentricity) o Cortical retinotopic map: cortical magnification (more for fovea/center) § Convergence of rods and cones: why is there cortical magnification? § Density of RGC encoding visual info decreases as you go from center to periphery § Ganglion cells representing central vision provide finer grained info (high res) from central vision o Receptive fields of simple v1 cells: edge detectors § Receptive fields become larger and more complex in concentric cells § Orientation selectivity and tuning (matters \ | - /) § Detect edges § Sensitive to specific orientation in specific position § Orientation “mapped” along cortex: preferred orientation changes as you move o Orientation columns § V1 neurons tuned to same orientation are stacked in vertical columns § Adjacent columns hav similar orientations, resulting in “pinwheels” o Complex v1 cells § Receptive fields larger and more complex § Oriented bar in general location (not as specific as in simple v1 cells) o Hierarchy of processing up to v1 § ‘building’ progressively complex representations as you move up the visual hierarchy § Photoreceptors à LGN à V1 § Size of receptive field increases as you go from V1 to v4 o ‘Preferred’ stimuli § We say a neuron codes for whatever makes it respond most § Simple v1 cells: '''Line orientation, in one spatial location / § '''RGC: a small point in space . § Complex v1 cells: line orientation, in a general location ///////// - Further processing in dorsal/ventral pathways o Dorsal: “where” o Ventral: “what” - Pigmented cell à Rods/cones à horizontal cell (integrates and adjusts for light/dark) àbipolar cellà amacrine cell (intercept RGC/bipolar; directional motion, light adaption) àRGC